1. Field of the Invention
This invention relates to a control circuit for a switched reluctance motor and, in particular, to a control circuit that uses discrete circuit components to control a motor that drives a relatively fixed load at a relatively fixed speed.
2. Disclosure of Related Art
A conventional switched reluctance motor (SRM) includes a stator having a plurality of pairs of diametrically opposed stator poles and a rotor having a plurality of pairs of diametrically opposed rotor poles. Windings or coils are typically disposed about the stator poles and the windings around any two diametrically opposed stator poles may be connected in series or in parallel to define one motor phase of the multiphase SRM. The windings associated with a motor phase may be referred to as a phase coil. By generating current through the phase coil, magnetic fields are established about the stator poles and a torque is produced that attracts a pair of rotor poles into alignment with the stator poles. The current in the phase coils is generated in a predetermined sequence in order to produce a constant torque on the rotor. The period during which current is provided to the phase coil--and the rotor poles are brought into alignment with the stator poles--is known as the "active stage" or conduction interval of the motor phase. At a certain point--either as the rotor poles become aligned with the stator poles or at some point prior thereto--it becomes desirable to commutate the current in the phase coil to prevent a negative or braking torque from acting on the rotor poles. Once this "commutation point" is reached, current is no longer generated in the phase coil and the current is allowed to dissipate from the phase coil. The period during which current is allowed to dissipate from the phase coil is known as the "inactive stage" of the motor phase.
A conventional control circuit for a switched reluctance motor incorporates a microprocessor which generates signals that are used to control the level of current in each motor phase coil. Microprocessors, however, are relatively expensive and are designed for applications in which the motor is used to drive loads of varying magnitude and at varying speeds. Microprocessors are also relatively complex, requiring relatively large amounts of time, money, and effort to develop and implement.
There is thus a need for a control circuit for a motor that will minimize or eliminate one or more of the above-mentioned deficiencies.